It is well known to use fibre optic sensor for the measurement of acceleration of certain structures. Such fibre optic sensors may for example be configured as fibre optic interferometers.
In recent years there has been a growing interest in using optical fibre sensing techniques in marine seismic exploration surveys for the exploration and monitoring of hydrocarbon producing zones and reservoirs.
For these purposes it is common to use seismic cables towed behind a vessel or deployed on the ocean floor. Such cables typically include an array of accelerometers capable of detecting acceleration produced by acoustic waves being reflected from regions below the ocean floor.
As an example United Kingdom patent publication GB 2,455,930 describe a method of fabricating a fibre optic accelerometer in which a movable mass, a fixed element and a movable element that moves with the mass is first provided, after which an optical waveguide is wrapped as a sensing coil to complete the accelerometer.
United Kingdom patent publications GB 2,455,931 and GB 2,422,661 describe examples of cross-line accelerometers and in-line accelerometers, respectively, which may be produced according to the method described in GB 2,445,930. The accelerometers are intended for being coupled to any surface or structure subjected to acceleration to be sensed, and in particular for being disposed within sensor stations spaced along a seismic cable used to obtain an ocean bottom seismic (OBS) survey.
Another example in the prior art is U.S. Pat. No. 6,891,621 which describes a fibre optic based cross-axis accelerometer for determining the acceleration of a structure. It is described how optical fibre coils could be employed as support members for a rotationally supported housing including a mass. The optical fibre could be used as a part of an optical fibre interferometer.
In seismic sensors it is normally required to arrange a large number of sensors, such as accelerometers. GB 2,455,930 describes an effective method of manufacturing fibre optic accelerometers. Because that solution enables assembly of all of the main components of the accelerometer before the fibre is coiled onto the assembly the handling of the optical fibre is reduced and a reliable manufacturing process is enabled.
In the field of fibre optic sensing it is now common practice to package a plurality of fibre optic sensor elements together, for example as exemplified by the seismic station package of U.S. Pat. No. 7,551,517 in which three fibre optic accelerometers and one hydrophone are packaged into a seismic station.
International patent application publication WO 2007/042761 discloses a fibre-optic package which typically comprises two or more fibre optic accelerometers coupled together by fused-fibre coupling. A method of fabricating a fibre-optic package is also disclosed in which a step in the method is to form first, second and third individual fibre-optic accelerometers from a single optical fibre without the need for fibre-fusion splices between the sensors. In this method the individual fibre coils are mounted into the package after coiling.
With respect to manufacturing processes, the solution of WO 2007/042761 describes the coiling of the fibre coils prior to assembling of the package, inherently necessitating handling and mounting of the accelerometers with fibre coils and/or the cutting and splicing of the fibre of each fibre coil of the package.
Unpublished European Patent Application No. 09172431.0 to this applicant describes the manufacturing of fibre optic sensors in general and in particular the manufacturing of seismic sensors for use in seismic surveying activities in subsea environments in which a continuous optical fibre is coiled onto two or more sensor elements so as to form multiple sensor units. The sensor elements are arranged as part of a stack in a coiling jig. The sensor elements are disassembled from the stack prior to being mounted in a sensor package, as shown on FIG. 5 of that application.
Due to the large number of sensors in seismic cables it is desirable to minimize the handling of each sensor and to be able to perform the manufacturing process in as few steps as possible. In fibre optic based seismic cables a simplification of the process of assembling the optical fibre based accelerometers and a reduction of the handling of the optical fibre is particularly desirable. Seismic cable design and optical fibre accelerometer designs that could help to meet such objectives would be particularly preferable.